1. Field of the Invention
The present invention relates to oscillators and, more particularly, to voltage controlled surface acoustic wave ("SAW") oscillators.
2. Description of Related Art
As is known in the art, there exists a need in many radio frequency applications, such as in radar receivers and transmitters, for highly stable oscillators or other sources of radio frequency signals. One of the more general approaches to fulfilling this need is to use a so-called SAW stabilized oscillator in which a SAW device such as a delay line or a resonator is disposed within a feedback loop which has an integral number of 2 pi radians of phase shift and excess small signal gain at a particular frequency within the passband frequency of the SAW stabilizing device.
In many of these applications, it is necessary to have an oscillator the output frequency of which is highly stable both over long periods of time, as well as over relatively short periods of time. The first stability characteristic is generally referred to as the aging characteristics of the oscillator and is principally related to long-term variations in the electrical characteristics of the devices comprising the SAW stabilized oscillator. More specifically, one of the major contributors to variations in oscillator performance over long periods of time may be the SAW device itself. SAW devices typically have aging rates on the order of several parts per million per year, although SAW resonators are now available having aging rates on the order of a few parts per million per year or less. Thus, aging is increasingly becoming a negligible cause of frequency instability in such devices.
Another cause of frequency instability is short-term frequency variations or noise. SAW stabilized oscillators have relatively poor close-to-carrier phase noise characteristics. That is, there exists a relatively large amount of noise power per unit of bandwidth at offset frequencies close to the carrier or fundamental frequency of the oscillator. This results principally from inherent characteristics of the SAW stabilizing element and other oscillator components such as the loop amplifiers.
A concept, besides noise, that plays a major role with respect to the background of the present invention is that of jitter, which may be time, amplitude, frequency, or phase related. Jitter refers to abrupt, spurious variations in the duration, magnitude, frequency, or phase of the frequency modulation of an interval, successive cycles, or successive pulses of a repetitive wave. Stated more simply, variations in the pulse positions within a sequence of pulses result in jitter. Some of the causes of jitter are dependent on the pulse pattern being transmitted while others are not. For example, random forms of jitter are caused by noise, interference, and mistuning of clock circuits. Pattern-dependent jitter results also from clock mistuning, from amplitude-to-phase conversion in clock circuits, and from intersymbol interference ("ISI"), which alters the position of the peaks of the input signal according to the pattern.
It is known that jitter accumulation over a digital link may be reduced by buffering the link with an elastic store and clocking out the digit stream under the control of a highly stable phase-locked loop. The elastic store is effectively a buffer that is initialized to hold a certain number of bits, and which expands and contracts to keep the bit length constant. The phase-locked loop is a circuit that uses feedback to minimize the deviation from one bit time to the next. It generally accomplishes this function by synchronizing a variable local oscillator with the phase of a transmitted signal.
With specific reference again to SAW oscillators, it is known to address some of the aforementioned needs and potential problems by including in the SAW oscillator loop a voltage controllable phase shifter which can introduce a relatively small amount of phase shift into the loop and thus adjust the frequency of oscillation of the loop or lock the frequency of oscillation to a desired frequency.
Heretofore, frequency control of SAW oscillators has been effected by means of varactor diodes. This solution suffers a disadvantage in that the component count of such frequency control systems is high, and thusly, production cost is also high.
Frequency control of various other types of oscillators has also heretofore been effected. For example, frequency control of crystal oscillators by means of a voltage controlled phase shifting network is known. However, these control systems are relatively complex. For example, the systems shown in U.S. Pat. Nos. 4,571,558 and 4,646,033 involve two integrator circuits for phase shifting purposes, and they operate the crystal in its series resonance. In any event, however, such teachings have in no way heretofore been applied to SAW oscillators.
Based upon the foregoing, it should be clear that it is a shortcoming and deficiency of the prior art that here has not yet heretofore been developed a relatively simple and inexpensive system and method for controlling the frequency of SAW oscillators.